"Doppler shift" refers to the stretching of waves (known as "redshifting") from a source moving away from the observer, and the compression of waves (called "blueshifting") from a source approaching the observer. Redshifted waves have a longer wavelength (or lower frequency) than they otherwise would, and blueshifted waves have a shorter wavelength (or higher frequency). This works with radio waves, light, and all other forms of electromagnetic radiation. It's also noticeable in sound waves-it causes a siren coming toward a listener to have a higher pitch than a siren moving away. Just as police equipped with radar guns use Doppler shift to determine the speed of a car, scientists can use the phenomenon to determine the speed at which a spacecraft or celestial object is moving toward or away from Earth.